1. Field of the Invention
This invention is an example of the general field of coated abrasives, which consist of a plurality of abrasive grit particles distributed over at least one major surface of and adhered to a flexible backing material.
In one specific embodiment, this invention relates to the provision of coated abrasives which can accomplish lens fining in a single step. The term "fining" is an established term of ophthalmic art.
Originally, lens fining with coated abrasives was divided into two steps requiring different coated abrasives for each step. Much commercial use is still made of this two step process, but in recent years at least one type of commercial product capable of accomplishing in a single step what had previously required two steps has been introduced. The prior art product of this type known to the applicants is believed to have only a single grits containing layer with one type of abrasive grits therein.
This invention in its preferred embodiments also relates to the field of adhesives curable by exposure to ultraviolet (hereinafter UV) light.
2. Technical Background
Further descriptions of the the fining process and of suitable machinery for accomplishing it are readily available in prior patents, e. g., U.S. Pat. Nos. 4,320,599 to Hill et al. and 3,732,647 to Stith, the entire specifications of which patents are hereby incorporated herein by reference. A properly designed coated abrasive can be an advantageous type of lapping tool such as is shown as lapping surface 78 of FIG. 2 of the Stith patent, as has been known in general terms heretofore.
In prior art coated abrasives known to the applicants, the abrasive grit particles are normally size graded, which means that grits with sizes greater or lesser than the average or median size for the particular abrasive article by more than a selected ratio are excluded as thoroughly as is practical from the collection of grits making up the article. Normally, all the grits in a single size graded coated abrasive article are part of a single population, so that samples of grits from different areas of the article or from different layers within the depth of the grits coating on the article will have the same distribution of grit sizes as the entire article, within the normal level of statistical variation for random samples of different sizes from the same population.
The use of adhesives capable of rapid cure under the influence of actinic radiation, particularly UV light, has provided attractive combinations of manufacturing speed and adhesive quality in many coating operations, including a wide variety of decorative surface coating, in which relatively thin and transparent adhesive coatings are adequate. Nevertheless, the use of UV cured coating materials for coated abrasives has been very limited. It appears to have been generally believed that the relatively thick layers of adhesives typically required for coated abrasives would be very difficult or impossible to cure with UV light, because of the limited depth of penetration of such light into most appropriate adhesive formulations. Therefore, most of the workers in the field are believed to have concentrated instead on electron beam curing, as exemplified by U.S. Pat. No. 4,547,204 of Oct. 15, 1985 to Caul. Electron beam curing, while effective, requires significantly greater capital investment than curing with UV light and presents a more serious potential hazard to personnel.
German Offenlegungsschrift No. 1956810 published July 21, 1971 purports to describe processes for making coated abrasives with adhesives cured by UV light, but appears to be purely speculative and non-enabling. No working examples are given, and the exposure times suggested are so impractically long--30-300 seconds--that the probable result of trying the suggested process would be the thermal destruction of the backing.
The only published enabling example of a coated abrasive prepared by UV curing known to the applicants is in Japanese Laid-Open Application No. 119491/1978, dated Oct. 18, 1978. This document indicates that the presence of an isocyanate compound in the adhesive is important for success with UV light initiated cure of adhesives for coated abrasives. Furthermore, although it was generally asserted in this Japanese publication that all the formulations disclosed therein are suitable for cure by UV light as well as electron beam curing, only one of the sixteen specific examples actually used UV light, and the adhesive used for this example contained no triacrylated monomers and only a little diacrylated monomer, with the bulk of the adhesive being non-acrylic types of polymerizable unsaturated esters and styrene. The main goal of the art described in this publication appeared to be the use of electron beams with lower than normal energy and of relatively inexpensive adhesives.
A waterproof paper coated abrasive with fast curing adhesives was disclosed in U.S. Pat. No. 4,047,903 to Hesse et al., but this product was cured by electron beam radiation only.